(1) Field of the Invention
This invention relates to a CT apparatus having an electromagnetic emitter for emitting electromagnetic waves in a conical form, and a planar detector, the emitter and detector being helically revolvable about an object under examination to scan the object. More particularly, the invention relates to a technique of suppressing artifacts appearing in CT images (computed tomographic images).
(2) Description of the Related Art
An X-ray CT apparatus may be cited as one example of CT apparatus that emits electromagnetic waves in a conical form and detects transmitted electromagnetic waves with a planar detector. Such an X-ray CT apparatus will be described below with reference to FIGS. 1 and 2.
As shown in FIG. 1, a conventional X-ray CT apparatus includes, opposed to each other across an object under examination M, an X-ray tube 51 for emitting an X-ray beam CB in a conical form, and a panel type X-ray detector 52 having a two-dimensional detecting surface 52a for detecting transmitted X rays. As shown in FIG. 2, the X-ray tube 51 and panel type X-ray detector 52 are movable about and relative to the object M. That is, a helical scanning is carried out by following a helical or spiral path SP and advancing along the body axis Z. As the X-ray tube 51 emits an X-ray beam CB in a conical form during a helical scanning operation of X-ray tube 51 and panel type X-ray detector 52, CT image composing data is collected from the X-ray detector 52. In this apparatus, the panel type X-ray detector 52 has numerous X-ray detecting elements arranged in a matrix form, and transmitted X-ray beams are detected in numerous detection lines succeeding one after another along the object""s body axis. As a result, numerous slice images may be acquired from an area of interest Ma at a time, thereby reducing the time consumed in photography.
Further, an X-ray CT apparatus of this type has been proposed that collects from the panel type X-ray detector 52 CT image composing data covering a 180xc2x0 scanning range for each point in the area of interest Ma (M. Defrise et al. xe2x80x9cA solution to the long-object problem in helical cone-beam tomographyxe2x80x9d Phys. Med. Biol. 45 (2000) 623-643). The apparatus having the above construction is also called the PI-line detection area type. As shown in FIG. 3, when a cylindrical surface SQ including the helical path SP of X-ray tube 51 is regarded as a detection area, a projection data collection area is between helical curves (arcs uuxe2x80x2 and ddxe2x80x2) less than a pitch of helical path SP as seen from the X-ray tube 51. Since, in this case, the site-dependent weight function in time of reverse projection is simplified to speed up an image reconstructing process.
To supplement the above, when an X-ray beam CB in a conical form is used, the apparatus becomes the three-dimensional CT type which, unlike the usual two-dimensional CT type, complicates the relationship between each point in the area of interest and X-ray detection element, and complicates the site-dependent weight function in time of reverse projection. However, as shown in FIG. 3, when collecting from the panel type X-ray detector 52 CT image composing data covering a 180xc2x0 scanning range from a scan position xs through a scan position xo to a scan position xe for each point P in an area of interest, there is an advantage that the relationship between each point in the area of interest Ma and X-ray detection element is relatively simple to simplify the site-dependent weight function in time of reverse projection. It will be appreciated that, for expediency of description, point P is shown as lying on the body axis Z in FIG. 3.
However, the conventional X-ray CT apparatus of the PI-line detection area type noted above has a disadvantage that artifacts tend to appear in a CT image ultimately obtained.
That is, with the PI-line detection area type, as shown in FIG. 4, artifacts could appear in the direction of a segment extending between scan positions xs and xe at opposite ends of the 180xc2x0 scanning range for each point in the area of interest of object M. In principle, the same data should be obtained by emitting an X-ray beam CB in a conical form from opposite directions to point P in the area of interest. However, these data actually are not necessarily the same because of the non-parallelism of beam elements of X-ray beam CB and the polychroism of X rays. As a result, artifacts are created in reconstructing CT images.
This invention has been made having regard to the state of the art noted above, and its primary object is to provide an X-ray CT apparatus of the PI-line detection area type which is capable of suppressing artifacts appearing in CT images ultimately obtained.
The above object is fulfilled, according to this invention, by a CT apparatus for making a plurality of helical scans about an object under examination placed on a support table, and collecting, through each scan, CT image composing data covering a scanning range of 180xc2x0 for each point in an area of interest, the CT apparatus comprising:
an electromagnetic wave emitting device for emitting electromagnetic waves in a conical form toward the object;
a planar detecting device opposed to the electromagnetic wave emitting device across the object for detecting the electromagnetic waves emitted from the electromagnetic wave emitting device and diverging two-dimensionally;
a drive device for moving the electromagnetic wave emitting device and the planar detecting device relative to the object while rotating the electromagnetic wave emitting device and the planar detecting device about the object, to cause electromagnetic waves to make helical scans around the object, whereby CT image composing data covering a 180xc2x0 scanning range for each point in the area of interest of the object are collected from the planar detecting device;
a plural scan control device for controlling the drive device such that the plurality of helical scans made of the object by the electromagnetic wave emitting device have helical paths with a phase difference therebetween of 360xc2x0 divided equally; and
an image reconstructing device for performing an image reconstructing process based on CT image composing data collected through the plurality of helical scans.
In a tomographic process performed by the above CT apparatus, the plural scan control device executes a plurality of helical scans successively of an object under examination. At this time, as electromagnetic waves (e.g. X rays) are emitted in a conical form during each helical scan, CT image composing data covering a 180xc2x0 scanning range is collected for each point in the area of interest. That is, this CT apparatus is the PI-line detection area type, in which the relationship between each point in the area of interest and each X-ray detection element of the planar detecting device is relatively simple to simplify a site-dependent weight function in time of reverse projection.
Further, since the helical scans are made to follow helical paths having an equi-sectional phase difference therebetween, CT image composing data is collected from a plurality of directions for each point in the area of interest through all the helical scans. That is, CT image composing data covering a 360xc2x0 scanning range is acquired ultimately. The CT image composing data acquired is transmitted to the image reconstructing device to carry out an image reconstructing process based on the CT image composing data collected through all the helical scans and covering the 360xc2x0 scanning range. By using the CT image composing data collected from a plurality of directions for each point in the area of interest, a discrepancy due to the non-parallelism of beam elements and the polychroism of beams is eliminated, whereby images are reconstructed properly. This suppresses artifacts appearing in X-ray CT images ultimately obtained.
In the CT apparatus according to this invention, the plural scan control device, preferably, is arranged to control the drive device to execute the helical scans to and fro by alternately reversing a scanning direction and a rotating direction for each scan. With this construction, the plural scan control device controls the drive device to execute the helical scans to and fro by alternately reversing the scanning direction and rotating direction for each scan. That is, after a helical scan from a starting end to a terminal end of the area of interest, a next helical scan is carried out from the terminal end back to the starting end, with the rotating direction also reversed. Since there is no need to perform a return scan between the two helical scans, images may be picked up in a short time.
In the CT apparatus according to this invention, the plural scan control device, preferably, is arranged to control the drive device to execute the helical scans to and fro and constantly in the same direction of rotation. With this construction, the plural scan control device controls a continuous photographic process with reciprocal helical scans in the same rotating direction, or a photographic process with helical scans in the same rotating direction and including a return scan in between. Thus, there is no need to switch the rotating direction for each helical scan.
In the CT apparatus according to this invention, the plural scan control device, preferably, is arranged to control the drive device to execute a non-helical, simple rotating scan in a range of xcfx80 to 2xcfx80 at each of opposite ends of each helical scan. With this construction, a non-helical, simple rotating scan takes place in a range of xcfx80 to 2xcfx80 at each of the opposite ends of each helical scan. Thus, sufficient CT image composing data is collected even at the opposite ends of each helical scan for which data tends to be insufficient.
In the CT apparatus according to this invention, the plural scan control device, preferably, is arranged to control the drive device to execute the helical scans having a phase difference therebetween of 360xc2x0 divided equally by an even number or an odd number. With this construction, an image reconstructing process is carried out properly based on the CT image composing data acquired. This suppresses artifacts appearing in X-ray CT images ultimately obtained.
In the CT apparatus according to this invention, the plural scan control device, preferably, is arranged to control the drive device to execute the helical scans in a range of xcfx80 multiplied by a multiple of 2. With this construction, image data may be picked up from an increased area of interest.
In the CT apparatus according to this invention, the plural scan control device, preferably, is arranged to control the drive device to execute a pre-scan before starting, and a post-scan after finishing, each helical scan effective to collect CT image composing data, the pre-scan and the post-scan being ineffective to collect CT image composing data. With this construction, the X-ray emitting device and planar detecting device may have a stable scanning speed when starting each scan for collecting CT image composing data. A post-scan carried out after each scan is effective to decelerate the X-ray emitting device and planar detecting device gradually, thereby to reduce the load applied to the apparatus.
In another aspect of this invention, a CT apparatus is provided for making a plurality of helical scans about an object under examination placed on a support table, and collecting, through each scan, CT image composing data covering a scanning range of 180xc2x0 for each point in an area of interest, the CT apparatus comprising:
an electromagnetic wave emitting device for emitting electromagnetic waves in a conical form toward the object;
a planar detecting device opposed to the electromagnetic wave emitting device across the object for detecting the electromagnetic waves emitted from the electromagnetic wave emitting device and diverging two-dimensionally;
a drive device for moving the electromagnetic wave emitting device and the planar detecting device relative to the object while rotating the support table, to cause electromagnetic waves to make helical scans around the object, whereby CT image composing data covering a 180xc2x0 scanning range for each point in the area of interest of the object are collected from the planar detecting device;
a plural scan control device for controlling the drive device such that the plurality of helical scans made of the object by the electromagnetic wave emitting device have helical paths with a phase difference therebetween of 360xc2x0 divided equally; and
an image reconstructing device for performing an image reconstructing process based on CT image composing data collected through the plurality of helical scans.
In a tomographic process performed by the above CT apparatus, the plural scan control device executes a plurality of helical scans successively, following helical paths having an equi-sectional phase difference therebetween. Thus, CT image composing data is collected from a plurality of directions for each point in the area of interest. CT image composing data covering a 360xc2x0 scanning range is acquired. The image reconstructing device carries out an image reconstructing process properly based on the CT image composing data collected through all the helical scans and covering the 360xc2x0 scanning range. This suppresses artifacts appearing in X-ray CT images ultimately obtained.